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Abstract. The presence of hydrogen in aluminium poses problems to the foundry and casting industries, because high residual hydrogen contents in molten aluminium cause significant porosity in the solid aluminium after casting. This usually renders the product useless as it may fail mechanically. Therefore, fast, accurate and reliable techniques are required for monitoring the dissolved hydrogen content in molten aluminium, but this particularly harsh environment places considerable restrictions on the equipment that can be used. Several methods are available for the determination of hydrogen in aluminium melts, but they either suffer problems of accuracy, reliability and longevity or are not applicable to industrial environments. It is considered that the most appropriate device for the hydrogen analysis in aluminium melts should be an electrochemical sensor, which employs a proton conducting solid electrolyte in conjunction with a measuring electrode and a suitable reference electrode. The electromotive force of such a cell allows direct calculation of the hydrogen concentration in the melt. However, all the electrochemical sensors reported in the literature thus far exhibit distinct drawbacks. This article discusses the various techniques for the determination of hydrogen in molten aluminium with particular emphasis on the benefits and shortcomings of the existing electrochemical sensors.

A new solid-state hydrogen reference electrode has been developed that is based on the two-component two-phase mixture of b-zirconium and d-zirconium hydride, and is suitable for use in conjunction with the high-temperature proton-conducting CaZr 0.9 In 0.1 O 3-d solid electrolyte. Coulometric titration studies have confirmed the presence of a true two-phase plateau, existing over a wide composition range, which may be exploited as a precision thermodynamic buffer for reference hydrogen partial pressure. Cell voltage measurements have demonstrated that potentiometric hydrogen sensors incorporating this reference electrode exhibit Nernstian response over a broad range of temperature and hydrogen partial pressure, as well as excellent thermal cycling and long-term stability. The new solid-state hydrogen reference electrode is of considerable technological relevance and has already found application in a commercialised sensor unit.

Indium-oxide-substituted calcium zirconate of the nominal composition CaZr 0.9 In 0.1 O 3−␦ is an important perovskite-type hightemperature proton conductor. In the present study, the transition from proton conduction to oxide ion conduction in this material has been investigated experimentally. A specially designed galvanic cell was employed, in which the oxygen chemical potential at the electrodes is determined by appropriate metal/metal-oxide mixtures while the hydrogen chemical potential is fixed by a gas atmosphere of known hydrogen partial pressure. Through cell voltage measurements at different temperatures proton and oxide ion conduction were discriminated, and conditions were identified under which virtually pure oxide ion conduction occurs despite the presence of a significant hydrogen partial pressure. Conductivity measurements allowed the determination of the activation energy of oxide ion conduction. The impact of a variable oxide ion vacancy concentration on the ion conducting properties of the solid electrolyte is discussed.